1. Field of the Invention
The present invention is directed generally to decontamination of cementitious surfaces. More specifically the present invention is related to a microbiological process for removing radioactive contamination from a concrete surface.
2. Relevant Technology
In the nuclear industry, concrete is commonly used to retain radioactive substances. There are thousands of nuclear facilities comprising about 50 square miles of radionuclide-contaminated contaminated concrete surfaces. These nuclear facilities use concrete ponds, canals, sumps, and other structures for the containment, transport, and storage of liquid and solid radioactive materials, resulting in the contamination of the concrete surfaces with radionuclides. Typically, the contamination is securely fixed on, or within one or two millimeters of the surface. Before the concrete is disposed of in a suitable waste disposal site, it is desirable to decontaminate the radio-active concrete surface.
One conventional method for decontamination entails dismantling the entire structure. However, all of the rubble from a dismantled structure must be disposed of as radiological waste even though the contaminated portions may comprise only the outer few millimeters. Another option involves manually chipping off only the outer few millimeters of the contaminated concrete. By either method, the cost to decommission and decontaminate the contaminated sites is estimated to be tens of billions of dollars.
In response to the significant need for an alternative cost-effective method for decontaminating a cementitious surface, U.S. Pat. No. 5,414,196 by Jennings, the entire disclosure of which is hereby incorporated herein by reference, is directed to microbial degradation of contaminated cement surfaces. The method comprises applying a microorganism, such as a species of thiobaccillus, to degrade the surface of the concrete and thereby release a material comprising the radioactive substance. The released material is then removed from the surface by vacuum suction, scraping, brushing, or abrasion blasting. Once the concrete surface has been sufficiently decontaminated, the process may be stopped by heating or by depriving the microorganism of essential nutrients or moisture.
However, these same parameters (temperature, nutrients, and moisture) must be maintained at a level conducive to the growth of the bacteria in order for this method to perform effectively. For example, it is preferable that the cementitious surface contain a nutrient source such as sulphur. If the concrete does not contain sufficient nutrient, an alternate source of nutrient needs to be applied. Further, the needed moisture level must be maintained by periodic, manual addition of water.
Though such addition of moisture and nutrients is successful at maintaining active microbial influenced degradation of the concrete surfaces, manual addition is impracticable as the final method for maintaining the process under commercial application. Not only does this increase the cost and burden associated with decontaminating the various structures, it also increases the potential health and safety risks of the personnel responsible for the manual addition of moisture and nutrients.
It is therefore an object of the present invention to provide a method for maintaining microbial influenced degradation without the need for manual application of nutrients or moisture.
It is another object of the invention to provide a practical, cost effective, and environmentally acceptable method for decontaminating concrete.
A further object of the invention is to provide a method for decontaminating large surface concrete structures, hot cells, ponds, canals, sumps, biological shields, and other structures requiring decontamination through a naturally occurring microbial process.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.
To achieve the forgoing objects, and in accordance with the invention as embodied and broadly described herein, the present invention relates to a new and useful method for microbial influenced degradation of contaminated structures. The preferred method of the present invention is a biomediated process for removing contamination from a cementitious surface utilizing encapsulated microorganisms for the microbial influenced degradation. The encapsulated microorganisms of the present invention do not require the manual addition of moisture or nutrients to maintain conditions conducive to growth. Thus, the encapsulated microorganisms are simply sprayed or painted on to the contaminated surface and left for a period of time sufficient for microbial influenced degradation activity to degrade the cementitious surface to a selected depth.
The process preferably comprises applying a substantially uniform coating of the encapsulated microorganism to a contaminated cementitious surface. The microbial influenced degradation activity is preferably permitted to continue until the cementitious surface has been degraded to a desired depth. It has been demonstrated that radioactive contamination is usually localized in the first 1-2 mm of cementitious material. The degraded layer is preferably removed by brushing, washing or scrubbing the degraded cementitious surface.